15 CFR 80.4: Signature of persons unable to sign their name.

A person requesting Census information who is unable to sign his or her name shall make an “X” mark where signature is required, and the mark must be witnessed by two persons who know the applicant. They must also sign the application certifying the applicant's identity. In the case of such persons who are unable to make an “X” mark, Census information can be released upon receipt of a physician's sworn statement verifying the disability and the written request of a parent, brother, sister, child or a spouse.